Voltage control of magnetism in Fe3-xGeTe2/In2Se3 van der Waals ferromagnetic/ferroelectric heterostructures

We investigate the voltage control of magnetism in a van der Waals (vdW) heterostructure device consisting of two distinct vdW materials, the ferromagnetic Fe3-xGeTe2 and the ferroelectric In2Se3. It is observed that gate voltages applied to the Fe3-xGeTe2/In2Se3 heterostructure device modulate the magnetic properties of Fe3-xGeTe2 with significant decrease in coercive field for both positive and negative voltages. Raman spectroscopy on the heterostructure device shows voltage-dependent increase in the in-plane In2Se3 and Fe3-xGeTe2 lattice constants for both voltage polarities. Thus, the voltage-dependent decrease in the Fe3-xGeTe2 coercive field, regardless of the gate voltage polarity, can be attributed to the presence of in-plane tensile strain. This is supported by density functional theory calculations showing tensile-strain-induced reduction of the magnetocrystalline anisotropy, which in turn decreases the coercive field. Our results demonstrate an effective method to realize low-power voltage-controlled vdW spintronic devices utilizing the magnetoelectric effect in vdW ferromagnetic/ferroelectric heterostructures.

The structure of the FGT/IS heterostructure device is characterized with cross-sectional TEM.The FGT and IS show layered structures with vdW gaps (Fig. S1a).The IS layer shows atomic alignments typical of a downward polarized monocrystalline 2H α-IS (Fig. S1b) 1,2 .We also see the existence of a thin (~2 nm) and uniform layer consisting of lighter elements at the interface between FGT and IS (Fig. S1a).This interfacial layer is repeatedly and consistently observed in other similar FGT/IS heterostructure specimens that we fabricated and measured.
We use EDS to identify and quantify elements across the interface (Fig. S1c).The interfacial layer has detectable quantities of carbon, silicon, and oxygen, all of which are also the full composition of PDMS.It may look as if the interfacial layer is mixed with FGT and IS according to the EDS graph (Fig. S1c), but judging from the clear boundaries of each layer (Fig. S1a), we believe it results from the relatively low resolution of the EDS measurement.
Therefore, we suspect it is the residue from the vdW material transfer process using PDMS.We speculate that this interfacial layer might act as an adhesive mediator of strain across the interface.The consistent voltage-induced Raman peak redshifts (in-plane tensile strain) in both the FGT and IS (see main text Fig. 3) confirm this.The voltage effect does not show any significant FGT-thickness-dependence in the thickness range 10.5~20 nm.This implies that the voltage-induced-piezo-strain transfer between the IS and FGT is effective up to ~20-nm-thick FGT layers.Note that in all the heterostructure devices studied, FGT thickness (10.5~20 nm) << IS thickness (50~100 nm).In the case the polarization-induced IS surface charge is the origin of the VG-induced Hc modulation, the effect would show a more obvious FGT-thickness-dependence.
For FGT thickness > 30 nm, the magnetic stripe domain phase appear 3,4 , with characteristic spilt M-H loops which makes it difficult to define Hc, and hence, only thin (< 20 nm) FGT layers are used for this study.Unlike the VG-induced redshift seen in main text Fig. 3, zero-bias Raman peak positions have little dependency on the VG applied prior to the zero-bias measurements.This implies the absence of strain at zero bias, i.e., the voltage effect is non-remanent, supporting the nonremanent Hc change observed in Fig. S3.Fig. S10.MAE/Fe (in meV) as a function of in-plane lattice constant for various doping concentrations.As explained in the main text, the FGT sample we used in the experiments correspond to the 1h/f.u.case.The in-plane lattice constants in the plot are chosen corresponding to the in-plane strain between -1.5% and 1.5% relative to the experimental lattice constant, which is the fourth data-point for each doping concentration plot.There is overall decrease in MAE as hole doping increases.We find that the dependence of MAE with respect to in-plane strain changes with hole doping.For doping smaller than 0.8h/f.u.MAE increases (decreases) with tensile (compressive) strain whereas for the doping larger than 0.8h/f.u.MAE shows opposite behaviour.For 0.8h/f.u.MAE decreases both for tensile and compressive strain.
We note that for doping less than 1h/f.u.MAE values maintain positive.However, for 1h/f.u.doping, the decrease in MAE is about 65% reduction in MAE for 0.5% tensile strain (a = 3.97 Å) and even induces sign change in MAE from out-of-plane to in-plane anisotropy for 1% tensile strain (a = 3.99 Å).

Fig. S1 .
Fig. S1.The cross-sectional TEM images and EDS graphs of a FGT/IS heterostructure device.

Fig. S4 .
Fig. S4.Non-remanent voltage effect on MOKE hysteresis loops with VG off, on, and then off

Fig. S5 .
Fig. S5.Reversibility test of VG-dependent Hc change in FGT/IS device measured over 50

Fig. S9 .
Fig. S9.Raman shifts at zero bias after voltage application.a) A series of zero-bias Raman

Fig. S11 .
Fig. S11.Structural and magnetic properties of FGT/IS heterostructure devices fabricated by